The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting a polishing endpoint during a chemical mechanical polishing operation.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a patterned stop layer, and planarizing the filler layer until the stop layer is exposed. For example, a conductive filler layer may be deposited on a patterned insulative stop layer to fill the trenches or holes in the stop layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing disk pad or belt pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time-consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
More recently, in-situ optical monitoring of the substrate has been performed, e.g., with an interferometer or reflectometer, in order to detect the polishing endpoint. Unfortunately, noise in the signal generated by the optical monitoring system may make it difficult to determine the proper endpoint.
In one aspect, the invention is directed to a computer-implemented endpoint detection method for a chemical mechanical polishing operation. In the method a first endpoint criterion, a first time window for the first endpoint criterion, and a second endpoint criterion are stored. A signal is received from a polishing endpoint detection system, and the signal is monitored for the first endpoint criterion. If the first endpoint criterion is not detected within the first time window, polishing is stopped at a default polishing time. On the other hand, if the first endpoint criterion is detected within the first time window, the signal is monitored for the second endpoint criterion and polishing is stopped if the second endpoint criterion is detected.
Implementations of the invention may include one or more of the following features. A second time window for the second endpoint criterion may be stored, and polishing may stop at a default polishing time if the second endpoint criterion is detected before the second time window. If the second endpoint criterion not detected, polishing may stop at the end of the second time window. A third endpoint criterion and a third time window for the third endpoint criterion may be stored. The third time window may be located before the first time window. If the third endpoint criterion is not detected within the third time window, polishing may be stopped at a default polishing time. A detection time at which the first or second endpoint criterion is detected may be stored. The default polishing time may be modified if the second endpoint criterion is detected within the second time window. The first time window may be modified if the first endpoint criterion is detected within the first time window. The endpoint detection system may optically monitors the substrate. The polishing operation may polish a metal or dielectric layer on the substrate. The endpoint detection method may be performed after the first endpoint criteria is detected in the first time window and the second endpoint criteria is detected in the second time window for a plurality of consecutive substrates.
In another aspect, the invention is directed to a computer-implemented endpoint detection method for a chemical mechanical polishing operation. In this method a series of N endpoint criteria are stored, each endpoint criteria associated with a time window. N is equal to or greater than 2. A signal is received from a polishing endpoint detection system, and the signal is monitored for the series of endpoint criteria. Whether an endpoint criterion is detected within the associated time window is determined, and if the endpoint criterion is detected within the associated time window, the signal is monitored for the next endpoint criterion in the series. The determining and monitoring steps are iterated until one of the endpoint criterion is not detected within the associated time window or a last endpoint criterion is detected within the associated time window. If the former, polishing is stopped at a default polishing time. If the later, polishing is stopped based on detection of the last endpoint criterion.
In another aspect, the invention is directed to a method of chemical mechanical polishing. In the method, a substrate is brought into contact with a polishing surface, and relative motion is created between the substrate and the polishing surface. A first endpoint criterion, a first time window for the first endpoint criterion, and a second endpoint criterion are stored. A signal is received from a polishing endpoint detection system, and the signal is monitored for the first endpoint criterion. If the first endpoint criterion is not detected within the first time window, polishing is stopped at a default polishing time. On the other hand, if the first endpoint criterion is detected within the first time window, the signal is monitored for the second endpoint criterion and polishing is stopped if the second endpoint criterion is detected.
In another aspect, the invention is directed to a method of chemical mechanical polishing. In the method, a substrate is brought into contact with a polishing surface, and relative motion is created between the substrate and the polishing surface. A plurality of endpoint criteria, a plurality of time windows and a default polishing time are stored. Each endpoint criterion is associated with one of the time windows. A signal from an endpoint detection system is monitored for the endpoint criteria. The times at which the endpoint criteria are detected are stored. Polishing stops after one of the default polishing times or detection of a last of the endpoint criteria. At least one of the default polishing time and the time windows is adjusted based on at least one of the times at which an endpoint criterion was detected.
Implementations of the invention may include one or more of the following features. The adjusting step may occurs if each endpoint criterion is detected in its associated time window. The adjusting step may include setting the default polishing time equal to a time of detection of the last of the endpoint criteria. The adjusting step may includes setting the default polishing time equal to an average of detection times of the last of the endpoint criteria in a plurality of substrates. The adjusting step may include setting a time window based on a detection time of an endpoint criterion associated with the time window. The setting step may includes calculating a start time and end time from the detection time and a margin. The margin may be a preset percentage or the margin may be determined from a difference between detection times of two endpoint criteria. The adjusting step may be performed after each endpoint criteria is detected in its associated time window for a plurality of consecutive substrates.
In another aspect, the invention is directed to a chemical mechanical polishing system. The system has a polishing surface, a carrier head to hold a substrate in contact with the polishing surface, a motor coupled to one of the polishing surface and carrier head to generate relative motion between the polishing surface and substrate, an endpoint monitoring system, and a controller. The controller is configured to store a first endpoint criterion, a first time window for the first endpoint criterion, and a second endpoint criterion, to receive a signal from the monitoring system, to monitor the signal for the first endpoint criterion, to stop polishing at a default polishing time if the first endpoint criterion is not detected within the first time window, and to monitoring the signal for the second endpoint criterion and stop polishing when the second endpoint criterion is detected if the first endpoint criterion is detected within the first time window.
Implementations of the invention may include one or more of the following features. The endpoint monitoring system may direct a light beam to impinge a surface of the substrate during polishing. The controller may be configured to store a second time window for the second endpoint criterion. The controller may be configured to stop polishing at a default polishing time if the second endpoint criterion is detected before the second time window. The controller may be configured to stop polishing at the end of the second time window if the second endpoint criterion not detected.
In another aspect, the invention is directed to a computer-implemented control method for a chemical mechanical polishing operation. In the method, a first criterion, a first time window for the first criterion, and a second criterion are stored. A signal is received from a polishing endpoint detection system, and the signal is monitoring for the first criterion. If the first criterion is not detected within the first time window, a polishing parameter is changed a default polishing time. If the first criterion is detected within the first time window, the signal is monitored for the second criterion and the polishing parameter is changed if the second criterion is detected.
In another aspect, the invention is directed to a computer-implemented endpoint detection method for a chemical mechanical polishing operation. In the method, an endpoint criterion and a time window for the endpoint criterion are stored. A signal is received from a polishing endpoint detection system, and the signal is monitored for the endpoint criterion. Polishing is stopped at a default polishing time if the endpoint criterion is detected before the time window. If the endpoint criterion is detected within the time window, polishing is stopped when the endpoint criterion is detected.
Implementations of the invention may include the following features. Polishing can stop at an end of the time window if the endpoint criterion is not detected before the end of the time window.
Implementations of the invention may include zero or more of the following possible advantages. The endpoint detection procedure can be more robust and less likely to fail. Polishing can be stopped with reasonable accuracy even if the optical endpoint detector fails. The endpoint detection technique can be applied to both metal and oxide polishing. Overpolishing of the substrate can be reduced, and throughput can be increased.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.